This invention relates to an improvement of a linear stepper motor of the variable reluctance type.
Linear stepper motors of this VR type are often utilized in electronic-controlled typewriters. On the slider of a linear stepper motor of this kind a print head must be mounted, and the whole weight of the slider and the print head inevitably becomes fairly large. And the slider takes considerably long time, when it is halted at a designated position, before being completely stopped. It is sometimes liable to fall into missynchronization or pull-out, i.e., to lose harmony with the drive pulses. The above-mentioned tendency has also been problematical in a VR type linear stepper motor wherein a slider having four slider poles is moved by means of 2-2 phase excitation. The structure and excitation sequence in a conventional VR type linear stepper motor of four-slider-pole system are in a style described hereunder.
A stator is provided with a plurality of stator teeth formed with an equal pitch p along a longitudinal direction thereof. A slider including four slider poles, A pole, B pole, C pole, and D pole arranged in a longitudinal direction in that order is opposed to the stator with slider teeth formed on each of the slider poles by the identical pitch in the stator teeth. On those slider poles four windings are wound independently so as to make up A phase, B phase, C phase, and D phase in that order. In the structure of a conventional VR type linear stepper motor the distances separating the B pole, C pole and D pole from the A pole are determined to (L+1/4)p, (M+2/4)p and (N+3/4)p wherein L, M and N are natural numbers. For incrementing or stepping the slider in this stepper motor by 1/4 pitch by means of the 2-2 phase excitation, the slider poles must be excited by pairing of (1) A phase-B phase, (2) B phase-C phase, (3) C phase-D phase, and (4) D phase-A phase. In this type excitation, halting of the slider arises a large problem, laying aside advantageous excitation pairing of B phase-C phase and D phase-A phase, in the excitation pairing of A phase-B phase and C phase-D phase, wherein the slider vibrates largely to take much time before being completely stopped or even sometimes lose harmony with the drive pulses. This problem is caused by a leftwardly offset location of the magnetic flux for the slider, when the slider is attempted to stop at the position corresponding to excitation pairing of A phase-B phase, and by a similar rightward offset thereof when the slider is stopped at the position of C phase-D phase excitation. Reasons for that are surmised to be such that formation of a magnetic flux offset to right or to left is liable to cause vibration in the slider perpendicular to the moving direction thereof due to imbalance of the attracting magnetic force as a consequence of the offset, and this vibration incessantly varies in turn a fine gap between the stator teeth and the slider teeth, being accompanied by another vibration in the moving direction of the slider, so as to prevent the slider from being stopped rapidly and exactly. In cases of excitation at the pairing of B phase-C phase and D phase-A phase, the magnetic flux becomes symmetrical right-and-left for making the attracting force well balanced. In short, the structure and the excitation sequence, limitted thereto by that structure, of the conventional VR type linear stepper motors have not been free from the problems of large vibration of the slider when stopped hindering the speedy halting of the slider.